The invention refers to a prior art such as emerges, for example, from the publication entitled "Vaccum-ultraviolet lamps with a barrier discharge in inert gases" by G. A. Volkova, N. N. Kirillova, E. N. Pavlovskaya and A. V. Yakovleva in the Soviet journal Zhuranl Prikladnoi Spektroskopii 41 (1984), No. 4,691-695, published in an English-language translation of the Plenum Publishing Corporation, 1985, Doc. no. 0021-9037/84/4104-1194, $08.50, pages 1194 ff.
For high-power radiators, in particular high-power UV radiators, there are various applications, such as, for example, sterilization, curing of lacquers and synthetic resins, flue gas purification, and destruction and synthesis of specific chemical compounds. In general, the wavelength of the radiator has to be very precisely matched to the intended process. The most well known UV radiator is presumably the mercury radiator, which radiates UV radiation of the wavelength 254 nm and 185 nm with high efficiency. In these radiators, a low-pressure low discharge is struck in a noble-gas/mercury vapour mixture.
The previously mentioned publication entitled "Vacuum ultraviolet lamps . . . " describes a UV radiation source based on the principle of the silent electrical discharge. This radiator comprises a tube of dielectric material with rectangular cross section. Two oppositely situated tube walls are provided with two-dimensional electrodes in the form of metal foils which are connected to a pulse generator. The tube is sealed at both ends and filled with a noble gas (argon, krypton or xenon). Under certain conditions, such filling gases form so-called excimers when an electrical discharge is struck. An excimer is a molecule which is formed from an excited atom and an atom in the ground state. EQU Ar+Ar*.fwdarw.Ar.sub.2.sup.*
It is known that the conversion of electron energy into UV radiation with these excimers takes place very efficiently. Up to 50% of the electron energy can be converted into UV radiation, the excited complexes living only for a few nanoseconds and emitting their bonding energy in the form of UV radiation when they decay. Wavelength ranges:
______________________________________ Noble gas UV radiation ______________________________________ He.sub.2 * 60-100 nm Ne.sub.2 * 80-90 nm Ar.sub.2 * 107-165 nm Kr.sub.2 * 140-160 nm Xe.sub.2 * 160-190 nm ______________________________________
In the known radiator, the UV light produced in a first embodiment penetrates the outside space via an endface window in the dielectric tube. In a second embodiment, the wide sides of the tube are provided with metal foils which form the electrodes. At the narrow sides, the tube is provided with cutouts over which special windows through which the radiation can emerge are glued.
The efficiency achievable with the known radiator is in the order of magnitude of 1%--that is to say, far below the theoretical value of around 50%, because the filling gas heats up unduly. A further inadequacy of the known radiator is to be seen in the fact that its light exit window has only a compartively small area for stability reasons.
European application 87109674.9 dated 6.7.1987, Swiss application 2924/86-8 dated 22.7.1986 or U.S. application Ser. No. 07/076926 dated 22.7.1986 proposed a high-power radiator which has a substantially greater efficiency, which can be operated with higher electrical power densities and whose light exit area is not subject to the restrictions mentioned. In addition, in the generic high-power radiator, both the dielectric and also the first electrodes are transparent to the said radiation, and at least the second electrodes are cooled. This high-power radiator can be operated with high electrical power densities and high efficiency. Its geometry can be matched, within wide limits, to the process in which it is used. Thus, in addition to large-area flat radiators, cylindrical ones which radiate inwards or outwards are also possible. The discharges can be operated at high pressure (0.1-10 bar). Electrical power densities of 1-50 kW/m.sup.2 can be achieved with this construction. Since the electron energies in the discharge can be largely optimized, the efficiency of such radiators is very high, even if resonance lines of suitable atoms are excited. The wavelength of the radiation can be adjusted by means of the type of filling gas--for example, mercury (185 nm, 254 nm), nitrogen (337-415 nm), selenium (196, 204, 206 nm), xenon (119, 130, 147 nm), and krypton (124 nm). As in other gas discharges, the mixing of different types of gas is recommended.
The advantage of these radiators is in the two-dimensional radiation of large radiation powers with high efficiency. Almost the entire radiation is concentrated in one or a few wavelength ranges. In all cases, an important feature is that the radiation can emerge through one of the electrodes. This problem can be solved with transparent, electrically conducting layers or, alternatively, also by using, as the electrode, a fine-mesh wire gauze or deposited conductor tracks which, on the one hand, ensure the supply of current to the dielectric, but which on the other hand, are largely transparent to the radiation. It is also possible to use a transparent electrolyte (for example, H.sub.2 O) as a further electrode, and this is advantageous for the irradiation of water/sewage since, in this manner, the radiation produced penetrates the liquid to be irradiated directly, and this liquid also serves as coolant.
Such radiators radiate only in a solid angle of 2 .pi.. Since, however, every element of volume situated in the discharge gap radiates in all directions (i.e., in a solid angle of 4 .pi.) one half of the radiation is initially lost in the radiator described above. It can be partially recovered by skillfully fitting mirrors, as was already proposed in the reference cited. In this connection, two things have to be borne in mind:
any reflecting surface has, in the UV range, a coefficient of reflection which may be markedly less than 1; and PA1 the radiation thus reflected has to pass three times through the absorbing quartz glass.